Atomic layer volatilization process for metal layers

ABSTRACT

A two-stage method to remove a metal layer from a substrate surface comprises using a CMP process to remove a first portion of the metal layer from the substrate surface, and using an ALV process to remove a second portion of the copper layer from the substrate surface. The ALV process comprises pulsing a co-reactant into a reactor housing the substrate, wherein the co-reactant reacts with the metal layer to form a volatile metal-containing product, and then evacuating the reactor to volatize and remove the metal-containing product.

BACKGROUND

During the manufacture of integrated circuits on semiconductor wafers,chemical mechanical polishing (CMP) processes are often used toplanarize layers of deposited material. For instance, when trenches andvias are etched into a dielectric layer and filled with copper metalduring a dual damascene process, a CMP process may follow to polish awayany excess copper that has deposited onto the surface of the dielectriclayer.

Typical CMP processes suffer from various drawbacks. The CMP process cancause dishing and erosion of the metal layer. Some areas of the metallayer may become over-polished while other areas become under-polished,causing the surface topography of the metal layer to be highly uneven.This uneven topography may accumulate as additional metal layers aredeposited may lead to variations of final metal dimensions andelectrical performance across the integrated circuit.

Since the polishing is uneven over the surface of the wafer,conventional CMP methods require that the wafer be intentionallyover-polished to ensure that all of the metal is removed from thesurface of the dielectric layer. Unfortunately, over-polishing resultsin the loss of metal within the features and the loss of a portion ofthe dielectric layer, yielding trenches and vias with lower metal volumeand decreased aspect ratios.

Other drawbacks include reduced selectivity between the metal and thedielectric material due to the mechanical component of the CMP process,difficulty in ascertaining when a predetermined thickness has beenreached when polishing down a layer, contamination due to wettingissues, accidental overexposing layers of material, and the force of theCMP process causing scratches or damage to the surface of asemiconductor wafer. Accordingly, improved processes are needed for theremoval of excess metal from the surface of a semiconductor wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a two-stage process for removing metal from a substrate inaccordance with an implementation of the invention.

FIGS. 2A to 2F illustrate structures formed when the process of FIG. 1is carried out.

FIGS. 3A to 3C illustrate how an ALV process may be used to open up apinched-off feature.

DETAILED DESCRIPTION

Described herein are systems and methods of removing metal from thesurface of a wafer. In the following description, various aspects of theillustrative implementations will be described using terms commonlyemployed by those skilled in the art to convey the substance of theirwork to others skilled in the art. However, it will be apparent to thoseskilled in the art that the present invention may be practiced with onlysome of the described aspects. For purposes of explanation, specificnumbers, materials and configurations are set forth in order to providea thorough understanding of the illustrative implementations. However,it will be apparent to one skilled in the art that the present inventionmay be practiced without the specific details. In other instances,well-known features are omitted or simplified in order not to obscurethe illustrative implementations.

Various operations will be described as multiple discrete operations, inturn, in a manner that is most helpful in understanding the presentinvention, however, the order of description should not be construed toimply that these operations are necessarily order dependent. Inparticular, these operations need not be performed in the order ofpresentation.

Implementations of the invention provide a process by which metal may beremoved from the surface of a semiconductor wafer, such as the surfaceof an interlayer dielectric (ILD), with reduced over-polishing andunder-polishing issues. The methods described herein combine a bulk CMPprocess with a novel atomic layer volatilization (ALV) process toefficiently planarize a metal layer and produce a more even surfacetopography with reduced ILD loss. As such, subsequently deposited layersdo not exacerbate surface topography issues and the aspect ratio offeatures within the ILD layers is maintained.

FIG. 1 is a method 100 of removing a metal layer from a substrate inaccordance with an implementation of the invention. For theimplementations described herein, the metal layer consists of a coppermetal layer. It is to be understood, however, that in furtherimplementations, metals other than copper may be used. FIGS. 2A through2F illustrate structures formed when the method 100 is carried out.

The method begins by providing a substrate with a dielectric layer andan overburdened copper metal layer needing to be removed (102 of FIG.1). The dielectric layer includes at least one feature, such as a trenchor a via, that is filled with the copper metal. The copper layer hasgenerally been deposited using a process such as chemical vapordeposition (CVD), physical vapor deposition (PVD), atomic layerdeposition (ALD), electroplating (EP), or electroless plating (EL).Alternate deposition processes may have been used.

FIG. 2A illustrates a substrate 202 with a dielectric layer 204deposited thereon. The dielectric layer 204 includes a feature 206, suchas a trench or a via. An overburdened copper layer 208 is deposited onthe dielectric layer 204 and fills the feature 206. The overburdenedcopper layer 208 may have a thickness that is typical in the art. Forinstance, the thickness of the overburdened copper layer 208 may rangefrom 100 Angstroms (Å) to 5000 Å or more. The substrate 202 is generallyformed using a bulk silicon or a silicon-on-insulator substructure,although it may be formed using materials such as germanium, indiumantimonide, lead telluride, indium arsenide, indium phosphide, galliumarsenide, or gallium antimonide, any of which may be combined withsilicon. Materials that may be used to form the dielectric layer 204include, but are not limited to, silicon dioxide (SiO₂), carbon-dopedoxide (CDO), silicon nitride (SiN), organic polymers such asperfluorocyclobutane (PFCB), and fluorosilicate glass (FSG).

In accordance with implementations of the invention, a two-stagepolishing process is carried out to remove the copper metal from thesurface of the dielectric layer. The first stage is a bulk CMP processto remove a bulk portion of the overburdened copper layer (104 of FIG.1). The CMP process ends with little or no damage to the underlyingdielectric material, leaving a thin copper metal layer over thedielectric layer. The second stage is a novel ALV process that preciselyremoves the remaining copper metal from the dielectric layer withminimal damage to the dielectric material (106 of FIG. 1). Unlikeconventional CMP processes, the ALV process does not apply a downwardforce on the surface of the dielectric layer, thereby reducingmechanical defects such as scratches.

The bulk CMP process (104) may be a conventional polishing process. Asis well known in the art, a CMP process generally involves the use of arotating polishing pad and an abrasive, corrosive slurry on asemiconductor wafer. After a material such as copper metal is depositedon the surface of a semiconductor wafer, the polishing pad and theslurry physically grind flat the microscopic topographic features untilthe metal is planarized. Chemical reactions that take place between theslurry and the wafer surface further contribute to the planarizingprocess. In implementations of the invention, the metal is polished bythe polishing pad until the metal is reduced to a predeterminedthickness ranging from 20 Å to 500 Å. At this point, a thin copper metallayer remains covering both the dielectric layer and the metal-filledfeature. Since the dielectric layer has not been exposed, the thincopper metal layer protects the underlying dielectric layer from thedrawbacks of CMP processes described above.

In accordance with implementations of the invention, conventional CMPslurries may be used. For instance, some conventional CMP slurriesinclude abrasive particles such as silicon dioxide (SiO₂), aluminumoxide (Al₂O₃), or cerium oxide (CeO₂). The CMP slurries used may alsoinclude additional components, including but not limited to oxidizerspecies such as hydrogen peroxide (H₂O₂), organic complexing agents,surfactants with both hydrophobic and hydrophilic chemical groups,and/or corrosion inhibitors such as benzotriazole.

FIG. 2B illustrates the substrate 202 after the CMP process has beencarried out to remove a portion of the copper metal layer 208. As shown,the CMP process yields a relatively thin copper metal layer 208remaining on the dielectric layer 204. Although not shown in FIG. 2B, asurface topography of the thin copper metal layer 208 is generallyuneven.

Once the endpoint of the CMP process is reached, the method 100continues by using an ALV process to remove the thin copper metal layerthat remains on the dielectric layer (106). The ALV process 106 maybegin by providing the substrate in a reactor (process 106A). Thereactor may be the same type of reactor used in the art for ALD and CVDprocesses. In some implementations, the substrate may be heated withinthe reactor to a temperature between around 25° C. and around 450° C.and the pressure within the reactor may range from 0.01 Torr to 30.0Torr.

Next, a co-reactant is pulsed into the reactor (106B). In accordancewith implementations of the invention, the co-reactant is a materialthat is capable of reacting with the metal layer to form a volatilemetal-containing product. In some implementations of the invention, theco-reactant is an oxygen based co-reactant such as oxygen (O₂), ozone(O₃), or carbon monoxide (CO). In further implementations of theinvention, the co-reactant is a halide based co-reactant such ashydrogen chloride (HCl), hydrogen bromide (HBr), hydrogen fluoride (HF),bromine (Br₂), chlorine (Cl₂), or fluorine (F₂). In furtherimplementations of the invention, alternate oxygen-based or halide-basedco-reactants may be used.

In various implementations of the invention, the following processparameters may be used for the co-reactant pulse. The co-reactant pulsemay have a duration that ranges from around 0.05 seconds to around 90seconds with a flow rate that ranges from around 25 standard cubiccentimeters per minute (SCCM) to 500 SCCM, depending on hardware andprocess demands. The specific number of co-reactant pulses may rangefrom 1 to 1000 pulses. The co-reactant temperature may be between around25° C. and 300° C.

A carrier gas may be employed to transport the co-reactant, with atemperature that generally ranges from around 25° C. to around 300° C.Carrier gases that may be used here include, but are not limited to,argon (Ar), xenon (Xe), helium (He), hydrogen (H₂), nitrogen (N₂),forming gas, or mixture of these gases. The flow rate of the carrierwill vary based on hardware designs and process demands.

An RF energy source may be applied at a power that ranges from 5 W to2000 W and at a frequency of 13.56 MHz, 27 MHz, or 60 MHz. It should benoted that the scope of the invention includes any possible set ofprocess parameters that may be used to carry out the implementations ofthe invention described herein.

In accordance with implementations of the invention, the co-reactantreacts with the thin copper metal layer to convert a top portion of thecopper metal into a thin, volatile copper-containing product. Thethickness of the copper metal layer is therefore reduced since the topportion of the copper metal is consumed when forming the volatilecopper-containing product. Oxygen-based co-reactants such as O₂ and O₃generally form a volatile copper oxide layer with the compositionCuO_(n), where n is typically 1 or 2. Halide-based co-reactantsgenerally form a volatile copper halide layer with the compositionCuX_(n), where X is typically Cl, Br, or F and where n is typically 1 or2.

FIG. 2C illustrates the substrate 202 after the co-reactant has reactedwith a top portion of the copper metal layer 208 to form a copper oxideor halide layer 210. As shown, the copper oxide or halide layer 210consumes a portion of the copper metal layer 208, thereby reducing thethickness of the copper metal layer 208.

After the copper oxide or halide layer has been formed, the method 100continues by purging the reactor (106C). The purge gas may be an inertgas such as Ar, Xe, N₂, He, or forming gas and the duration of the purgemay range from 0.1 seconds to 60 seconds, depending on the ALV reactorconfigurations and other operating conditions. In most implementationsof the invention, the purge may range from 0.05 seconds to 10 seconds.In some implementations, this purge process may be omitted.

Next, in accordance with implementations of the invention, the reactoris evacuated to volatize the copper oxide or copper halide (106D). Thepressure within the reactor therefore is adjusted to a value between0.001 Torr to 29 Torr. The volatized copper oxide or copper halide liftsoff the surface of the copper metal layer and is removed from thereactor during the evacuation process. The copper metal layer is lefthaving a reduced thickness due to the removal of copper by the volatizedcopper oxide or copper halide. In some implementations, the purge andevacuation processes are carried out simultaneously.

FIG. 2D illustrates the substrate 202 after the reactor has beenevacuated and the copper oxide or copper halide layer 210 has volatizedand been removed. As shown, the copper metal layer 208 remains with areduced thickness due to the loss of copper metal by the ALV process.

The above described ALV process (106) may be repeated one or more timesto remove the copper metal layer in its entirety (108 of FIG. 1). Forexample, in some implementations, the ALV process may be repeated 2 to1000 times in order to remove all of the copper metal. The ALV processis a selective process that reacts with and volatizes the copper metalover the dielectric material. As such, the copper metal may be entirelyremoved from the surface of the dielectric layer without damage to orloss of the dielectric material. The ALV process of the inventiontherefore maintains metal in the trenches and vias and also maintainsaspect ratios.

FIGS. 2E and 2F illustrate the substrate 202 undergoing a subsequent ALVprocess to remove the remaining portion of the copper metal layer. FIG.2E illustrates the conversion of the remaining copper metal into acopper oxide or copper halide layer 210. FIG. 2F illustrates thesubstrate 202 after the copper oxide or copper halide 210 has beenvolatized and removed, leaving behind a dielectric layer 204 free ofcopper metal on its top surface. The feature 206 still includes itscopper metal and substantially maintains its aspect ratio since littleor no portion of the dielectric layer 204 was polished away.

In further implementations of the invention, the ALV process describedherein may be used to remove other metal layers, such as barrier layersand adhesion layers. As is known in the art, after a feature is etchedinto the dielectric layer during a dual damascene process, a barrierlayer is generally deposited over the dielectric layer and into thefeature to prevent copper metal from diffusing into the dielectricmaterial. The barrier layer is often formed of a metal such as tantalum,tantalum nitride, titanium, titanium nitride, tungsten, tungstennitride, cobalt, cobalt nitride, nickel, nickel nitride, ruthenium, orruthenium nitride. An adhesion layer is then deposited over the barrierlayer to enable the subsequent deposition of copper. The adhesion layeris generally formed of a metal such as tantalum, titanium, or ruthenium.The portions of the barrier layer and adhesion layer outside of thefeatures, namely on the surface of the dielectric layer, are typicallyremoved using a conventional CMP process.

In accordance with implementations of the invention, however, an ALVprocess may be used to remove the excess barrier and adhesion metals. Inan implementation of the invention, oxygen or halide based co-reactantsare pulsed into the reaction chamber to react with the metals used toform the barrier and adhesion layers. For instance, the oxygen-basedco-reactants may form tantalum oxide, titanium oxide, tungsten oxide,cobalt oxide, nickel oxide, or ruthenium oxide, while the halide-basedco-reactants may form a tantalum halide, a titanium halide, a tungstenhalide, cobalt halide, nickel halide, or a ruthenium halide. Fortantalum, titanium, and ruthenium, oxygen-based co-reactants generallyform a tantalum oxide, titanium oxide, or ruthenium oxide layer with thecomposition TaO_(n), TiO_(n), or RuO_(n) where n typically ranges from 1to 5. Alternately, an oxygen-based co-reactant such as CO may be used toreact with the barrier layer and/or adhesion layer to form a volatilecarbonyl compound containing ruthenium, tantalum, or titanium. Further,for tantalum, titanium, and ruthenium, halide-based co-reactantsgenerally form a tantalum halide, titanium halide, or ruthenium halidelayer with the composition TaX_(n), TiX_(n), or RuX_(n) where X istypically Cl, Br, or F and where n typically ranges from 1 to 5. Thepurge and evacuation process described above may then be used tovolatilize the tantalum, titanium, or ruthenium compounds and removethem from the surface of the dielectric layer.

In an alternate implementation of the invention, the ALV processdescribed herein may be used to open up features that have beenpinched-off during a metal deposition process. FIG. 3A illustrates afeature 300 formed in a dielectric layer 302 that includes a void 304that developed during the deposition of a copper metal layer 306. Due tothe high-aspect ratio of the feature 300, issues such as trench overhangoften lead to a pinching-off of the opening of the feature 300, whichresults in the formation of the void 304.

Turning to FIG. 3B, the ALV process described above may be used toconvert a top portion of the copper metal layer 306 into a copper oxideor halide layer 308. The process conditions and co-reactants describedabove may be used here. Next, as shown in FIG. 3C, a purge andevacuation process may be used to volatilize and remove the copper oxideor halide layer 308, thereby reducing the thickness of the copper metallayer 306. In this instance, the ALV process removes enough copper metalto open up the feature 300, as shown in FIG. 3C. In furtherimplementations, two or more ALV processes may be carried out to removeenough copper metal to open up the feature 300. A subsequent copperdeposition process may follow, such as electroplating, electrolessplating, CVD, or ALD, to fill the opened feature 300.

Accordingly, an atomic layer volatilization process has been describedto precisely remove metal in a stepwise process that overcomes theissues inherent in conventional CMP processes. The ALV processesdisclosed herein enable complete copper, barrier, and adhesion layerremoval between features without resorting to a CMP overpolishingprocess that is required in conventional CMP processes to ensurecomplete metal removal. The use of an ALV process to remove metal fromthe surface of a substrate produces flatter surface topography, betterwafer-scale uniformity, and reduced electrical RC variations compared toconventional CMP processes. Various interconnect metals, such as copper,tantalum, titanium, and ruthenium may be removed by the methodsdisclosed herein. Products such as CuCl, CuCl₂, TiCl₄, TaCl₅, and RuCl₃are volatile and will be removed as gaseous by-products.

The ALV process of the invention provides many advantages over aconventional CMP process. For instance, the ALV process is a gas-phaseprocess with no applied physical stresses (e.g. no downward pressure)that leads to good selectivity for copper removal and less scratchingdefects relative to CMP processes. The ALV processes of the inventionalso reduce erosion and dishing compared to conventional CMP processes.

Finally, the ALV processes described herein are compatible with existingsemiconductor processing equipment. The co-reactants (both oxygen- andhalide-based) have been used in furnace deposition and processing. Forexample, halide based ALD precursors (e.g., TiCl₄) are currently used insemiconductor processing.

The above description of illustrated implementations of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific implementations of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize.

These modifications may be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific implementationsdisclosed in the specification and the claims. Rather, the scope of theinvention is to be determined entirely by the following claims, whichare to be construed in accordance with established doctrines of claiminterpretation.

1. A two-stage method to remove a metal layer from a substrate surfacecomprising: using a CMP process to remove a first portion of the metallayer from the substrate surface; and using an ALV process to remove asecond portion of the metal layer from the substrate surface, whereinthe ALV process comprises: pulsing a co-reactant into a reactor housingthe substrate, wherein the co-reactant reacts with the metal layer toform a volatile metal-containing product, and evacuating the reactor tovolatize and remove the metal-containing product.
 2. The method of claim1, wherein the second portion of the metal layer has a thickness lessthan around 100 Å.
 3. The method of claim 1, further comprising purgingthe reactor before the reactor is evacuated.
 4. The method of claim 1,wherein the ALV process is repeated as necessary to substantially removethe metal layer from the substrate surface.
 5. The method of claim 1,wherein the co-reactant is an oxygen-based co-reactant.
 6. The method ofclaim 5, wherein the oxygen-based co-reactant comprises O₂ or O₃.
 7. Themethod of claim 1, wherein the co-reactant is a halide-basedco-reactant.
 8. The method of claim 7, wherein the halide-basedco-reactant comprises HCl, HBr, HF, Br₂, Cl₂, or F₂.
 9. The method ofclaim 1, wherein the metal is copper.
 10. The method of claim 9, whereinthe co-reactant comprises a material that can react with copper to forma volatile copper oxide with the composition CuO_(n), wherein n is 1 or2.
 11. The method of claim 9, wherein the co-reactant comprises amaterial that can react with copper metal to form a copper halide layerwith the composition CuX_(n), wherein X is CI, Br, or F and wherein n is1 or 2.